Sewage and fertilizer runoff into China's Lake Taihu have fed a nasty bloom: an annual explosion of frothy cyanobacteria, which release neurotoxins into the lake. Hans Paerl, a marine and environmental scientist who studies Lake Taihu, says the warmer temperatures brought by climate change only contribute to the slime's advance.

IRA FLATOW, HOST:

This is SCIENCE FRIDAY. I'm Ira Flatow. If you're just joining us, it's our annual Halloween program. And next up, an environmental villain. It's actually a sci-fi menace dreamed up a long time ago in film.

(SOUNDBITE OF FILM)

UNIDENTIFIED MAN: The distant stars. The lonely Earth. The 21st century and lurking beyond the strange immensity of conquered space, exploding in unspeakable horror, the green slime. The civilized world at war with alien forms, whose slimy touch means instant, horrible death. Invaders from beyond the stars, the Green Slime.

FLATOW: Ten points if you can name when that movie came out. I'm talking about toxic blue-green algae in our case. The filmmakers may have overstated its immediate danger. I think scientists are reasonably certain it doesn't come from beyond the stars. But as my next guest says, it is a real threat to environmental and human health and climate change may be making it worse.

Hans Paerl is a professor of marine and environmental sciences at the University of North Carolina at Chapel Hill's Institute of Marine Sciences in Moorhead City, North Carolina. Welcome to SCIENCE FRIDAY, Dr. Paerl.

HANS PAERL: Hi, Ira. How you doing?

FLATOW: Overdid it a bit in that movie?

PAERL: Well, you know, now I see how you fit the whole thing in.

(LAUGHTER)

FLATOW: The Halloween theme. And, yeah, it took a little work. Now, you just got back from China. I actually just got back from two weeks in China.

PAERL: Yeah. I was going to say ni hao.

FLATOW: Ni hao. You were studying an infestation of this blue-green slime, this algae. Tell us what you saw.

PAERL: Well, we've been working with the Chinese now for about four years on an NSF, US NSF, China NSF project that is geared towards, first of all, understanding how these organisms became as aggressive as they are in the lakes that we're working on. And secondly, how to manage them. How to get rid of them. I guess as you probably know, cyanobacteria, or blue-green algae, have been around for a long time.

They have been on Earth for about two billion years. They were the first organisms that figured out how to split water and produce oxygen by photosynthesis. And they've seen lots of changes that have occurred on Earth, including, you know, the advent of oxygen itself but warming, cooling, glacial periods, etc. They have a playbook that is very, very sophisticated.

And they are now taking advantage of many of the changes that we're imposing on the environment, including too many nutrients, climate change, particularly warming, hydrologic modifications - building dams, for example, to slow down the water coming into our ecosystems. So, you know, we're playing into their playbook, basically.

FLATOW: Yeah. Tell us what you found in particular in China.

PAERL: Well, the lake that we've been working on is called Taihu. It actually means Big Water in Mandarin. It is a lake that is the third largest lake in China. It is a huge lake. It's 100 kilometers across. And the lake has changed, basically, from a very desirable diatom dominated lake - I don't know if that means anything to you, but diatoms are good algae. Good food source. To one that now has these massive signs of bacterial blooms(ph). They're paint-like scums that cover the lake from one side to the other, basically, now for eight months out of the year. So the lake's basically gone through a state change and we're trying to figure out how much the nutrients need to be reduced that are causing this problem and which nutrients need to be reduced in order to basically knock it back to a more desirable state.

FLATOW: Yeah. Isn't it true that basically if you turned on your tap water you've got green slime...

PAERL: Yep.

FLATOW: ...coming out of it?

PAERL: Well, that's what happened one morning in 2007 in a couple of the cities that use the lake for drinking water. And by the way, about 15 million people use the - in the Taihu basin use the lake for drinking water. One morning everyone woke up with this green slime, smelly slime, coming out of their faucet. What had happened was that the scums were so extreme that they basically overwhelmed the drinking water plant for a city named Wuxi of about five million people and basically knocked the water supply out of action. And the central government and provincial governments had to come in and provide bottled water for about, well, almost 10 million people for about six weeks before they got things back into order. So this basically was a crisis that started and it got the Chinese government mobilized into starting to do something about their surface water supplies.

Because Taihu is not the only lake that is impacted by this problem.

FLATOW: Now, if I remember correctly from my visits around China, isn't that lake the source of a lot of the ornamental rock that the Chinese use in their gardens and the temples?

PAERL: That's correct. In fact, if you go to Beijing's famous temple there...

FLATOW: Saw it.

PAERL: You saw it.

FLATOW: Yeah.

PAERL: It's kind of this foamy looking rock.

FLATOW: Yeah.

PAERL: That is hauled out of Taihu and put into prestigious places everywhere in China.

FLATOW: And so they really respect at least the sources of the rock where it's coming from.

PAERL: Oh, yeah. Well, Taihu is a revered lake. It is a lake that the Chinese have valued for, I guess, thousands of years. It's a place where people like to come to, to rest and take their vacations now. There are lots of four star hotels around the lake. And with this bloom problem, of course, it's thrown that business, you know, tourism, recreational business, fisheries, and water supply into turmoil.

So there's, you know, lots involved in terms of economics. But also health problems.

FLATOW: Yeah.

PAERL: Because these algae-produced toxins that can lead to anything - well, they range from liver disorders and, you know, if you drink the water long enough, very serious ailments, including cancer. And they also produce neurotoxins that can give people neurological symptoms that range from, you know, tingling in the fingers to Alzheimer's-like symptoms.

FLATOW: Can you boil the water and get rid of...

PAERL: No. Unfortunately, these compounds that are produced - they're called cyclic peptides - are very strong compounds in terms of not being able to digest them with boiling. So the only way to get them out is to use activated charcoal filtration, which is very expensive. And, you know, for 10 million people that is basically impractical. Or UV oxidation to break up the bonds in the compound itself.

FLATOW: Now, you hear a lot of stories about how bad the air pollution is - I certainly saw it there when I was in Beijing - and other environmental catastrophes like this one, but not a lot about what the government's doing about it. Is the government working actively to clean up this lake and understand the problem?

PAERL: Well, that's sort of the good news part of this story, I think, and that is that because of Taihu's notoriety as being a valuable place, the government has really mobilized its research and management power into, you know, coming up with a nutrient management strategy that will hopefully reduce these blooms and eventually eliminate them.

Something on the order of I believe $5 billion - or equivalent of $5 billion U.S. has been promised by the government to, quote, clean up Taihu. And some of the brightest scientists in China are working with us to, you know, help formulate a nutrient management strategy for the lake.

FLATOW: And how pervasive is it around the whole country and around the rest of the world?

PAERL: Very. And I would say that doesn't just involve developing countries, but countries that are well developed that are simply generating more waste, nutrients coming from land, agricultural nutrients as well as waste water nutrients. So it's a problem that is basically present on every continent except maybe the Antarctic.

FLATOW: Huh. 1-800-989-8255 is our number. You can also tweet us @scifri. You had a commentary in the journal Science this week talking about how climate change is only going to exacerbate this problem.

PAERL: Yeah. Well, correct. One thing about blue-green algae, or cyanobacteria - they're really synonymous; the microbiologists like to call them cyanobacteria, the botanists blue-green algae - but these organisms, they basically grow, they basically are bacteria. If you look at them under the microscope, they have all the structural characteristics of bacteria. No organized nucleus, no organelles.

And they grow like bacteria. Which means that they actually optimize their growth at quite high temperatures, somewhere around body temperature for us, for example. So as things get warmer, their growth rates get closer to the optimal whereas the growth rates of the other algae, the ones we like to have in the lake, actually are past their optimum.

So, you know, cyanos like it hot, so to speak. They, you know, global warming is basically playing into their playbook in the sense that it's optimizing their growth. So when you combine more nutrients coming in with warmer conditions, that's really kind of the perfect combination for them to become more aggressive invaders. And we can see this in lakes, for example...

FLATOW: Yeah.

PAERL: ...in high latitudes, for example, northern Europe and even the tip of South America, where the ice now goes out earlier, for example. Or some lakes don't have any ice on them at all anymore in the wintertime. So the conditions for them to be able to take over and be aggressive are much earlier in the year and they last much longer in the year because it takes longer for the ice to come on.

So there's, you know, there's a good formula for them to expand. The other thing is that when we warm up surface waters, it creates a stronger density difference between the surface water and the bottom water. The bottom water stays relatively cool. So as you increase the surface water, you create a situation that's more resistant to wind mixing, which is something that they like too.

Because they like to be buoyant and right up at the surface so they can take in all the light they can and basically dominate the entire water column, shading out the other algae.

FLATOW: Well, they've had eons of practice at this.

PAERL: They have.

FLATOW: Yeah.

PAERL: And, you know, I - when I teach classes on this subject, I generally equate them with cockroaches because they're unwelcome guests that once they get into your house, or once they get into your lake, they're very difficult to get rid of. And what the article in Science also points out is that there are several feedback situations that occur when they get into an ecosystem.

They basically create their own microenvironment by dominating the surface waters, controlling the light that passes through the water, and then as they die eventually, they sink to the bottom and create conditions whereby the nutrients can get rereleased back up into the water column and they're ready to go the next year.

FLATOW: Wow. We've got to stop feeding them. Thank you very much, Dr. Paerl, for taking time to be with us today.

PAERL: Thanks, Ira.

FLATOW: Hans Paerl, professor of marine and environmental sciences at the University of North Carolina at Chapel Hill's Institute of Marine Sciences in Moorhead City. I'm Ira Flatow. This is SCIENCE FRIDAY from NPR.

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